Tri-tip can be hard to find around these parts. I got mine from Coborn's in Ramsey, but I don't know of a source in the city. There are a few things you can substitute, and one of the best would be flank steak. I think boneless shell steak, hanger steak, or london broil might work too--ask a butcher.
The trick is a short and flavorful marinade with some reserved for brushing on after the cooking. Cook the steak over high heat and then finish it over medium-low. I think that it tastes best cooked medium-well, which seems to be traditional in CA--it's really not a steak that's served rare. Also, the end pieces cook faster, which gives a fun variety of done-ness levels.
Pat the steak dry and place in a large zip-loc. Combine the marinade ingredients, reserving 1/4 c. for later.
1/2 c. soy sauce
1/3 c. oil
1/4 c. Worcestershire sauce
4 cloves garlic, crushed
2 Tbsp. red pepper paste (or 1 Tbsp. sriracha)
1/4 c. brown sugar
1 Tbsp. ground black pepper
2 Tbsp. chopped chives or green onion
Add the marinade to the bag and marinate for 1-2 hrs, turning once or twice. Grill over high heat for 3-5 minutes per side and then transfer to medium heat until cooked medium-well. You want the outside crisp and almost-burnt and the inside cooked through but not tough. Remove from heat and brush with reserved marinade. Cover loosely, and allow to rest 5 minutes. Slice thinly against the grain and pour any juices from the plate over the meat.